A technique of controlling a fan motor mounted to a vehicle with a host controller such as an electric control unit (“ECU”), for instance, is disclosed in patent literature 1, as one example of hitherto available motor control systems including such motor control devices. The host controller in this literature supplies a rotation speed command of a fan in a form of PWM signal to a brushless motor equipped with a drive control circuit. The drive control circuit thus rotates the fan at a rotation speed corresponding to a duty factor of the PWM signal. Besides, patent literature 2 discloses a structure in which a motor control unit outputs a rotation detection signal to a host controller, for instance, in addition to a rotation speed command in a form of PWM signal.
Incidentally, there is growing number of cases in recent years for such motor control systems that are controlled by digital signals using pulse signals. There increases a possibility with such digital processing to cause adverse influence to other apparatuses due to electromagnetic radiation of noises attributable to pulse signals while providing flexibility in the processing. In the case of the above motor control system mounted to a vehicle, for instance, electromagnetic noises are radiated from a main motor body, a power supply, control lines and the like which give rise to a risk of adverse influence to such devices as a radio mounted to the vehicle.
Certain means have been used to suppress the influence of noises of this kind, such as installing a noise eliminating circuit using capacitors and inductance elements, shielding a source of the noise emission, and providing a structure that enables the shortest possible routing of feeding lines and control lines. In addition, patent literature 3 discloses a method of reducing spurious electromagnetic emission in a vehicle by transmitting signals using a cable of twisted-pair structure to cancel out magnetic fields generated by the propagating signals.
In the methods for reducing electromagnetic emission such as those discussed above associated with the noise eliminating circuit, shielding means, and the cables of twisted-pair structure, however, there remain some drawbacks that they increase a number of the circuit components and shielding members for the noise preventive measures, and necessitate special cable materials like the twisted-pair cables. There is also a problem with the structure of shortening the feeding lines and control lines because they impose limitations on the mounting flexibility of the power supply, motor and the like.